Macrophage migration inhibitory factor (MIF) also known as glycosylation-inhibiting factor, L-dopachrome isomerase, or phenylpyruvate tautomerase is a highly conserved protein with pleiotropic actions. Discovered in the mid-1960s as a T cell cytokine that inhibited macrophage migration, its biochemical natures and its biological functions remained enigmatic for a long time. It is now well known that MIF (which exhibits tautomerase and oxidoreductase enzymatic activities) plays roles in cell growth, proliferation, and survival, as well as in leukocytic integrin activation, and induction of pro-inflammatory gene expression. In addition to MIF, a very recent study has identified a functional homologue of MIF with a similar genomic structure and expression patterns: the D-dopachrome tautomerase (DDT or MIF-2).
MIF and/or DDT are released upon stimulation by stress, endotoxin, inflammatory, and immune stimuli. Moreover MIF and/or DDT play an important, upstream role in the inflammatory cascade by promoting the release of other inflammatory cytokines (tumor necrosis factor (TNF)-α, interleukin (IL)-1, IL-6, IL-8, IL-12, interferon (IFN)-γ . . . ) and are potential therapeutic targets in multiple inflammatory, infectious, metabolic and autoimmune diseases including cancer. In patients with rheumatoid arthritis, MIF plays a central role in the activation of synoviocytes by increasing the expression of phospholipase A2, cyclooxygenase 2 (COX2), IL-6, IL-8, matrix metalloproteinase (MMP)-1 and MMP-3. In addition, MIF can modulate or “counter-balance” the anti-inflammatory and immunosuppressive effects of glucocorticoids on macrophages and T cells.
MIF and/or DDT bind to and activate CD74 and chemokine receptors CXCR2 and CXCR4. MIF signaling through CD74 occurs through two different modes: (a) in a CD44/Src dependent pathway in which CD74 interacts with CD44. CD44 is a transmembrane protein whose phosphorylation leads to activation of Src-family kinase and MAPK/ERK, PI3K/Akt and NF-κB pathway and to apoptotic resistance by increasing the anti-apoptotic factors BCL2, BCL-xL and by inhibiting p53; or (b) in a CD44 independent pathway in which CD74 cytosolic region is cleaved by a two-step process: translocation of CD74 cytosolic fragment (CD74-ICD) to the cell nucleus resulting in NF-κB activation, and induction of a survival cascade via up-regulation of BCL2. Moreover, CD74 is known to interact with angiotensin AT1-receptor and nitric-oxide synthase 2.
MIF is also implicated in multiple aspects of growth including control of cell proliferation and promotion of angiogenesis; moreover, an important role of MIF has been reported in tumor genesis. The inhibition of MIF-CD74 binding has been shown to reduce tumor growth and angiogenesis.
Considering the implication of MIF in various important diseases, this protein represents an interesting therapeutic target.
Injectable biological agents such as anti-cytokine antibodies or soluble cytokine receptors have been shown to inhibit MIF activities. However, these strategies present some drawbacks such as high cost and inconvenience of application.
In the past few years, significant efforts have been made to develop small molecules to inactivate MIF tautomerase activity, assessed using in vitro binding assay for MIF with CD74. Among identified products of interest, a prototypical MIF inhibitor (ISO-1) has been described and was shown to be active in vitro and in vivo:

However, ISO-1 has shown only micromolar potency and off-target effects have also been reported, limiting its usefulness (Lubetsky et al., J. Biol. Chem., 2002, 277(28), 24976-24982; Al-Abed et al., J. Biol. Chem., 2005, 280(44), 36541-36544; Meyer-Siegler et al., J. Immunol, 2006, 177(12), 8730-8739).
In 1999, Zhang and Bucala found that a group of dopachrome analogs could inhibit the tautomerase activity of MIF at concentrations tenfold less than substrates (Zhang and Bucala, Bioorg. Med. Chem. Lett., 1999, 9(22), 3193-3198).
MIF modulators are also disclosed in WO2010/021693, especially N-benzyl-benzoxazol-2-one compounds of general formula (i)
                wherein R1 and R2 represent preferably H, CH3, OCH3, CH2OH, F, or OH and Z1, Z2, Z3, Z4 and Z5 represent preferably H or OCH3.        
This kind of N-benzyl-benzoxazol-2-ones is also reported as MIF antagonists in Cournia et al., J. Med. Chem., 2009, 52, 416-424; Hare et al., Bioorg. Med. Chem. Lett., 2010, 20, 5811-5814; Xu et al., Drug Discov. Today, 2013, 18(11-12), 592-600.
The prior art molecules may not provide an optimized inhibition of MIF tautomerase activity.
Thus, substantial work remains to be done to provide new, better-tolerated and more powerful therapeutic small molecules to inhibit MIF tautomerase activity.
The Applicant surprisingly evidenced that isosteres of above N-benzyl-benzoxazol-2-ones have superior MIF inhibitory effects compared to current known MIF inhibitors. Especially, the Applicant hereby provides compounds of Formula I
                wherein X, Ar, R1, R2, R3 and R4 are as defined below.        
Compounds of Formula I are more selective and demonstrate significant biological improvements, relative to ISO-1 or benzoxazol-2-ones reported above.